Water soluble tooling materials for composite structures

ABSTRACT

The present invention relates to a low density, water-soluble coring and tooling material used for the fabrication of composite parts. One aspect of the present invention relates to a lightweight, strong composite coring material that can be easily shaped and removed from cured composite parts. Another aspect of the present invention relates to a lightweight, strong composite tooling material that is easily tailored to provide a specific coefficient of thermal expansion and thermal conductivity, thus providing a tooling material that can be matched to the composite structure and material being fabricated.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on, and claims the benefit of,co-pending U.S. Provisional Application Serial No. 60/274074, filed onMar. 7, 2001, and entitled “Water Soluble Tooling Material For CompositeStructures.”

[0002] The present invention was made with U.S. Government support undergrant Number N68335-01-C-0053 awarded by the Naval Air Warfare Center.Accordingly, the Government may have certain rights in the inventiondescribed herein.

FIELD OF THE INVENTION

[0003] The present invention relates to a novel coring and toolingmaterial for polymer composites. Particularly, the present inventionrelates to a low-density, water-soluble composite blend used to form acore material for the fabrication of composite parts. In addition, thepresent invention relates to a low density, water-soluble compositeblend used to form a tooling material, where the blend can be tailoredto provide a desired coefficient of thermal expansion and thermalconductivity, thus providing a tooling material that is compatible withthe composite material used to fabricate the structure.

BACKGROUND OF THE INVENTION

[0004] Composite components are increasingly being utilized in a varietyof applications due to their high strength-to-weight and highstiffness-to-weight ratios. One industry in which composite componentsare used is the aerospace industry. Initially, composite components werelimited to secondary structures such as floorboards and engine cowlingsdue to limited experience with designing composite structures. However,as the mechanics of composite materials became better understood andhigher quality materials were developed, their use increased as primaryaircraft components such as flaps, wing sections, and even as the entirefuselage.

[0005] Currently, there exist commercial aircraft that have a completelycomposite fuselage and wings made entirely from composite materials.Commercial airline manufacturers have increased their dependence uponcomposite materials to meet their ever-increasing demands for improvedefficiency and lower costs. Composite materials also are used inmilitary and defense applications, where the performance requirementsmay be even more demanding. A significant drawback to the use ofcomposite structures in aerospace applications, whether commercial ormilitary, is the complicated and expensive tooling that is required fortheir fabrication. Many different processes exist for the fabrication ofcomposite structures, and many different demands are placed upon toolingdesigns and materials. Typically, a composite structure is fabricatedusing either a closed or an open mold system. In a closed mold system,dimensional accuracy is required for both sides of the compositecomponent. A composite structure of this type would be, for example, anaileron or flap, of sufficient thickness to allow the desiredaerodynamic shape to be formed on both sides. Alternatively, an openmold process can be utilized to fabricate parts such as engine cowlingsbecause only one surface, the outer surface (thus, the mold surface), isof importance. With either mold system, the tool gives the compositestructure its final shape.

[0006] Tools for composite structures can be fabricated from a varietyof materials. However, several factors must be considered in the tooldesign. For instance, the coefficient of thermal expansion of the moldmaterial is of fundamental importance. As the tool is heated, it maychange shape at a different rate than the composite materials if thecoefficients of the tool and composite material are not similar enough.At elevated temperatures the composite material becomes rigid, whereas,when it is cooled, it will contract. The difference in the coefficientof thermal expansion of the composite and of the tool can creategeometrical inaccuracies as well as residual stresses.

[0007] Another important factor to consider is the thermal conductivityof the tool material. If the tool material has a low thermalconductivity, significant time can be spent simply getting sufficientheat to the composite part. Thus, curing irregularities can developbetween areas of thick and thin tooling. These irregularities alsotranslate into geometric inaccuracies and residual stresses.

[0008] Given these restrictions, tools for composite structures are mostoften comprised of steel, invar, aluminum, and carbon/BMI. With theexception of invar and carbon/BMI materials, the tooling materialsgenerally have a much higher coefficient of thermal expansion than thecomposite material being fabricated, and this expansion must beaccounted for in the mold design. Also, metal mold materials generallyrequire complex and time-consuming machining operations in order tocreate the tool surface, which further contributes to designcomplexities. For larger components, the time required to generate thesurface of the tool can become unacceptable. Additionally, it can bevery difficult to make any modifications to metal tooling once made, ifchanges to a part are subsequently identified. Thus, if part changes arerequired, it is often easier to make new metal tooling rather thanattempt to re-work the original tooling.

[0009] Although composite-tooling materials may seem ideal due to thematched coefficient of thermal expansion, such tooling requires anothercomplex composite component fabrication cycle for the tool itself.Furthermore, a higher processing temperature for the composite structurerequires higher cure temperatures for the tool material. Generally, thisresults in the use of thermoplastic tooling systems that are difficultand expensive to work with.

[0010] Use of mandrels made of polymeric binder compositions to formrocket motors, housings and other uniquely shaped items is known. Forexample, U.S. Pat. No. 6,325,958, which is incorporated by referenceherein, discloses methods of manufacture of a mandrel from a mixturethat includes water-soluble organic binders. More specifically, thepreferred binder comprises, poly (2-ethyl-2-oxazoline), derivatives ofpoly (2-ethyl-2-oxazoline) and mixtures thereof, along withpolyvinylpyrrolidone, derivatives and copolymers of polyvinylpyrrolidoneand mixtures thereof. Poly (2-ethyl-2-oxazoline), also referred to as“PEO” or “PEOx,” tends to be a relatively high cost component.Additionally, the functional properties of PEOx, such as its glasstransition temperature, may not be compatible with certain compositeformulations for the parts made using the mandrels.

[0011] Other conventional materials used for making tooling such asmandrels include eutectic salt, sodium silicate-bonded sand, andpoly(vinyl alcohol) bonded ceramic microspheres. These materials posecertain processing problems associated with removal of the materialsfrom the cured parts, as well as with the disposal of the materials.Eutectic salt mandrels are heavy (ρ>2 g/cc) and have high lineal thermalexpansion (α>6×10⁻⁵K⁻¹). Furthermore, salt mandrels are brittle and mustbe cast into the desired shape while molten to avoid machining them withdiamond tooling. Despite being soluble in water, eutectic salt mandrelsproduce corrosive, environmentally unfriendly waste streams when washedfrom the cured composite part. Sodium silicate-bonded sand mandrels arereadily washed from the cured composite and do not produce corrosivewaste streams. Unfortunately, silicate-bonded mandrels are heavy andbrittle, making them difficult to machine without resorting to diamondtooling. Mandrels made from ceramic microspheres bonded together bypoly(vinyl alcohol) have low densities and form relatively easily buthave a limited range of temperatures between which they can be used,because poly(vinyl alcohol) polymer binder becomes crosslinked above200° C., making it difficult to wash the mandrel from the cured part.

[0012] Thus, there remains a need for compatible, cost-effective,water-soluble compositions for use as coring and tooling materials inthe fabrication of composite parts.

SUMMARY OF THE INVENTION

[0013] The present invention offers alternative coring and toolingsystem and materials. The present invention offers novel low-cost coringand tooling materials for composite parts. Unlike conventional coringand tooling materials, the materials of the present invention arereadily soluble in water and can easily be washed away from the finishedpart. Furthermore, the coring and tooling materials can be used in themanufacture of a wide range of composite parts that can be cured athigher temperatures than heretofore possible.

[0014] Accordingly, an object of the present invention is to provide acomposite coring and tooling material that is cost-effective,environmentally benign, and water-soluble.

[0015] Another object of the present invention is to provide coring andtooling materials that can be easily shaped and subsequently removedfrom cured composite parts.

[0016] Yet another object of the present invention is to providecomposite coring and tooling materials that are strong and lightweightyet capable of withstanding high curing temperatures.

[0017] Furthermore, an object of the present invention is to providetooling materials that can be tailored to provide a specific coefficientof thermal expansion and thermal conductivity, thus providing toolingmaterials that can be matched to the composite structure beingfabricated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic flow chart illustrating the steps in themanufacture of a composite part in accordance with the presentinvention; and

[0019]FIG. 2 is a plan view of a mandrel made in accordance with theprocess of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention relates to novel water-soluble coring andtooling materials that can be used as forms in the fabrication ofcomposite parts, particularly those having complex geometries. Thematerials are lightweight, environmentally benign, and water-soluble,and the cost of the bulk starting materials is low. Composite partsfabricated with the coring and tooling material have a wide range ofapplications, such as automobile, aerospace, and biomedical prosthesis.

[0021] Referring to FIG. 1, there is illustrated a process for makingtooling material from a composite blend. Once formed, the toolingmaterial then can be used in the manufacture of composite parts. As usedherein, “tooling material” relates to any structure used in thefabrication of composite parts, such as a mandrel or core form, wherethe structure provides a support matrix for the composite part as it isbeing fabricated. For example, the tooling material may be used as aninternal core around which the part is formed. As another example, thetooling may be used as an external mold within which the part is formed.

[0022] In an initial step in the process, the composite blend used forthe tooling material is provided. Generally, the composite blendincludes a polymeric binder, water and, optionally, one or moreadditives selected to modify the physical properties of the binder andenhance the characteristics of the finished tooling material. Thecomponents are added to prepare a blend having a desired consistency.For example, the composite blend can be prepared as a slurry or as apaste, depending on the methods selected for forming the toolingmaterial and the properties desired for the finished tooling material.

[0023] The polymeric binder of the composite blend preferably is awater-soluble thermoplastic binder having high thermal stability.Water-soluble polymers such as polyvinylpyrrolinone (PVP) and blends orcopolymers thereof can be used as the thermoplastic binder. Preferablythe binder is PVP. PVP has a relatively high glass transitiontemperature (Tg). For example, the glass transition temperature of PEOis about 65° C., whereas the glass transition temperature of PVP isabout 190° C. The higher Tg increases the resistance of the driedtooling material towards slumping at higher curing temperatures, whichcould otherwise cause dimensional inaccuracies in the cured compositepart. It thus is possible to use the PVP-based tooling materials in thefabrication of a wide range of composite parts.

[0024] In preparing the composite blend, the thermoplastic binder ismixed with water to provide a solution. Additives can be mixed with thesolution as desired to provide the composite blend. Additives caninclude microspheres, plaster, metal particles, polyester orpolypropylene fibers, graphite and/or coke particles, compatibilizerssuch as alkali lignosulfonate, and mixtures thereof, which are selectedto enhance the functional properties of the tooling material.

[0025] The microspheres may be organic solids, metal or ceramicmicrospheres, or combinations thereof. Ceramic and metallic microspheresare preferred. The microspheres may be hollow or solid and are intendedto be small particles. Typically, the size of the microspheres isbetween about 10 to about 200 microns, although materials outside ofthis range are anticipated for use in the practice of the presentinvention. One suitable microsphere that can be used is commerciallysold under the name Extendospheres® SLG Grade microspheres by PQCorporation, Valley Forge, Pa. These microspheres are hollow ceramicmicrospheres with a mean sphere diameter of about 120 micrometers. Themicrospheres serve as a lightweight, low-density filler constituting themajor phase of the tooling material.

[0026] A material such as plaster can be used in the composite blend toimprove the castability of the blend when making the tooling material.The types of plaster that may be used include plaster of paris andgypsum plaster. Talc or similar material also can be added as a fillerto the composite blend to slow the rate of hardening of the compositeblend.

[0027] Metallic or high thermal conductivity ceramic fillers can beadded to enhance the thermal conductivity of the composite blend.Examples of high conductivity ceramic fillers include graphite, alumina,and silicon carbide. Various metallic powders having high thermalconductivities and low coefficients of thermal expansion can be used.Aluminum is one example of such a metallic filler. Aluminum flakes,aluminum tadpoles, and aluminum needles may serve as an aluminum filler.Generally, the type of particle selected will impact the amount ofmetallic filler that can be added to the blend. By way of example, it isexpected that a greater amount of aluminum tadpoles could be added ascompared to aluminum flakes.

[0028] Polyester or polypropylene fibers can be blended with the polymerbinder to enhance the compressive strength of the tooling material,particularly when higher curing temperatures are anticipated. Withincreasing temperatures and exposure times for curing of the compositeparts, it is desired to monitor the compressive strength of the toolingmaterials to avoid any undesired decreases in the compressive strengththat could result in distortion of the geometry of the part. Anysuitable type and form of polyester or polypropylene fiber that iscompatible with the binder and the composite part can be used. Examplesinclude chopped polyester or polypropylene or other types of syntheticfibers. Preferably, polypropylene fibers are used.

[0029] Graphite and coke can be added to the composite blend to increasethe thermal conductivity of the tooling materials. Examples of graphiteparticles include Type 4012 and Type A625 graphite from Asbury Graphite,Asbury N.J. Examples of coke include needle coke, such as Type 9019 fromSuperior Graphite Company, Chicago, Ill., and fluidized coke, such asGrade 4349 from Asbury Graphite, Asbury, N.J.

[0030] Addition of inorganic fillers typically requires use ofcompatibilizers or dispersants to maintain the particles in suspensionin the composite blend. Lignosulfonates are well known dispersants for awide variety of inorganic fillers. Furthermore, their high phenoliccontent enables them to readily form miscible blends with PVP due tostrong hydrogen bonding interactions present between the phenolichydroxyl group and the amide carbonyls present in the PVP polymerbackbone. Use of compatibilizers or dispersants may provide the addedbenefit of increasing the glass transition temperature of the compositeblend. Cross-linking of the dispersant and the polymer binder may resultin such an increase. It is expected that even a 5-10° C. increase in Tgcan result in a substantial enhancement of the heat stability of thetooling materials.

[0031] The blend can be a pourable slurry, moldable clay-like paste, oreven a solid. For a slurry, the viscosity ranges from between about 10⁵to about 10⁷ centipoise (cP) at room temperature. Moldable claystypically have viscosities of at least two orders of magnitude highercompared to pourable slurries. The composite blend is placed into a moldform so that it may be cast. The mold form typically includes means ofde-watering the composite blend. For example, the mold form may beconfigured to allow water to drain from the composite blend. That is,the mold form may have a screen along a bottom surface so thatde-watering is effected by draining water through the screen, either bygravity or by application of a partial vacuum.

[0032] The de-watered tooling material is removed from the mold form andsubjected to a drying operation. The drying can be carried out in anydrying oven at a temperature between about 100 to about 125° C. for atime sufficient to provide the desired degree of drying, which will varywith the thickness of the tooling material. A preferred drying cycleconsists of drying between about 100 to about 125° C. for one hour foreach inch of thickness of the material. If additives such asmicrospheres are used in the composite blend, the binder materialadsorbs onto the additives during the drying process, as well aspossibly during the prior blending step.

[0033] In an important aspect, the tooling material requires no complexprocessing in order to make mold having the desired shape. The toolingmaterial can be cast around a master part to create either an open orclosed mold. The tooling material also can be machined into the desiredform. Use of a combination of both methods also is possible.

[0034] The tooling material 10 is finished to obtain the desired shape.The tooling material 10 undergoes a minimal amount of shrinkage as thematerial cures. Once the tooling material surface has been achieved, thesurface finish can be repaired or polished using traditional techniques,as desired. Cracks or other undesired features in the surface may besmoothed over using a finishing composition 12 that is water soluble andwill not alter the properties of tooling material when used subsequentlyin fabricating the composite parts. Preferably, the finishingcomposition includes a polymer binder and plaster. The finishingcomposition also can include polyester or polypropylene fibers.Preferably, the finishing composition includes between about 2 to about10% PVP or PVP copolymer, between about 25 to about 50% plaster of parisand/or talc, between about 25 to about 50% water, and between about 0 toabout 2% polyester or polypropylene fibers. The finishing compositionpreferably will have a more viscous consistency so that it can beapplied to the outer surface of the tooling material and will adhere tothe outer surface without spreading or running off the surface. Theviscosity of the composition is between about 10⁶ to about 10⁷ cP.

[0035] The material will also have a consistency that is amenable tomachining with conventional tooling 14 as known to those of skill in theart. As an example, the machining may be accomplished with a lathe ormilling machine using carbide tooling, preferably at slower cuttingspeeds.

[0036] Preferably, the porosity of the dried tooling material is betweenabout 5 to about 15%. If the porosity of the tooling materials isgreater than desired, a water-soluble sealant also can be applied to theouter surface of the tooling materials once formed. The sealant willlimit migration of resin from the composite part into the toolingmaterial. As an example, the sealant can include between about 10 toabout 15 wt % PVP, between about 55 to about 65 wt % water and betweenabout 20 to about 30 wt % latex paint conditioner.

[0037] The finished tooling material then can be used in the manufactureof a molded composite product. For example, in the manufacture of amandrel, the molded core 10 of FIG. 2 may have an optional coating orinsulation 16 applied to the outer surface. A ribbon of fiber materialepoxy coating 18 may be wound on the molded core 10 to assume the shapeof the core 10 and form the composite product 20. The molded epoxycoating casing 20 is cured, for example, by application of heat orlight. It is noted that when using the cores of the present invention,it is possible to heat the epoxy coating to temperatures of at leastabout 550° F. without significant degradation of the core 10.

[0038] In an important aspect, the tooling materials are soluble inwater. With water-soluble tooling materials, the core 10 can be removedby flushing the core 10 with a solvent, preferably water. The waterbreaks down the core materials into the components of the blend, namelythe binder, which is water soluble, and any additives. The core 10 thusmay be removed from the engine casing 20. It is possible to obtaintooling materials that remain soluble in water even after exposure totemperatures of 550° F. or greater.

[0039] When the mold material is incorporated into the compositestructure, features like channels, recesses, integral stiffeners andhollow sections can be created with the mold material. Upon curing ofthe final composite part, the mold material in the channel or recess ofthe final part can simply be washed out, leaving the proper partgeometry.

[0040] There are numerous advantages associated with the construction asdescribed. For example, the materials are safe and easy to use becausethe binder is water soluble. The blend provides increased heat stabilityand creep resistance for the tooling materials. Additionally, the blendexhibits enhanced thermal conductivity and lower thermal expansion andgenerally will maintain the density of the tooling material uponheating.

EXAMPLES

[0041] The following examples further illustrate preferred embodimentsof the present invention but are not be construed as in any way limitingthe scope of the present invention as set forth in the appended claims.

Example 1

[0042] This example illustrates a composite blend for use as a core formfor the fabrication of composite parts. The coring material includes acomposite blend of hollow ceramic-microballons and a high thermalstability thermoplastic binder. In preparing the composite blend, thethermoplastic binder is mixed with water to form a first solution. Thefirst solution is subsequently mixed with a ceramic micro-sphere fillerto provide a composite blend in the form of a moist, formable paste. Thepaste can be shaped and dried in a drying oven at between about 100 toabout 125° C. for about 1 hour per inch of thickness. The dried pasteform can be subsequently machined as desired, thereby producing amandrel or core having a desired configuration. Examples of compositeblends containing PVP and ceramic microsphere filler are shown in Tables1 and 2. TABLE 1 Wt. (lbs.) Wt. % Solution PVP K90 0.24 15% Water 1.485% Total 1.60 100%  Paste Solution 1.60 20% Extendospheres SLG 6.40 80%Total 8.00 100% 

[0043] TABLE 2 Wt. (g.) Wt. % Solution PVP 14.06 15% Water 79.7 85%Total 93.75 100%  Paste Solution 10.00 20% Ceramic microspheres 40.0080% Total 50.00 100% 

[0044] Mandrels formed from the composite blend were fabricated bypressing the moist, formable paste into a molded shaped, drying theshaped part for 24 hours, sealing the dried part with silicone andfurther drying the part for 3 days. These mandrels were then used in anautoclave run as a preform. In the autoclave run, a S2/8551 glass/epoxyprepreg was used. A 15 psi vacuum, and an external pressure of 100 psi,was used, with the curing performed at 250° F. for 1 hour and 350° F.for 3 hours.

[0045] In a temperature range between 25° C. to 180° C., samplesprepared from the composite blend shown in Tables 1 and 2 were measuredto have a coefficient of thermal expansion of 5×10-6 mm/mm° C. However,slight shrinkage in the size of the samples occurred in a temperaturerange from between room temperature to 180° C. In order to eliminateshrinkage and obtain dimensional stability in the samples, the samplecan be subjected to an annealing treatment at the final curetemperature. For example, the samples were annealed at 190° C. for 1hour. After annealing, samples prepared from the composite blend shownin Tables 1 and 2 were measured to have a coefficient of thermalexpansion of −1.04×10-6 mm/mm° C.

Example 2

[0046] This example illustrates a composite blend for use as a toolingmaterial for fabrication of composite parts. The tooling materialcomprises a composite blend having a high thermal stabilitythermoplastic binder and either metal filler or high conductivityceramic filler. The metallic or ceramic fillers used in the compositeblend increase the overall thermal conductivity of the blend, and thus,provide a tooling material that can be tailored to provide specificvalues of thermal expansion and heat transfer. Conventional toolingmaterials, although inexpensive, are inferior due to their inability tohave tailored coefficient of thermal expansion and thermal conductivity.

[0047] High conductivity ceramic fillers, such as graphite, alumina, andsilicon carbide, can be used in the present invention. Tables 3 and 4illustrate composite blends containing PVP and graphite powder. Note,composite blends having graphite powder as the ceramic filler requiredispersants for the graphite powder. TABLE 3 Solution 1 Wt. (g.) Wt. %PVP K90 25% & Water 60.00 25% Water 180.00 75% Total 240.00 100%  BatchSize: 1900 cc Material Vol. % Density Wt. % Weight Solution 2 Water10.00% 1.00 10.37% 190.00 Lignosulfonate 0.25% 1.00  0.26% 4.75 GraphiteSpheres 89.75% 0.96 89.37% 1637.04 Total 100.0%   100% 1831.79 PasteSolution 1 12.00% 1.00 12.2% 228.00 Solution 2 88.00% 0.98 87.8% 1638.56Total 100.0%  100% 1866.56

[0048] TABLE 4 Solution 1 Wt. (g.) Wt. % PVP & Water 50.00 25% Water150.00 75% Total 200.00 100%  Batch Size: 1900 cc Material Vol. %Density Wt. % Weight Solution 2 Water 10.00% 1.00 4.7% 190.00 Dispersant0.25% 1.00 0.1% 4.75 Graphite Spheres 89.75% 2.25 95.2%  3836.81 Total100.0%  100%  4031.56 Paste Solution 1 12.00% 1.00  6.4% 228.00 Solution2 88.00% 2.00 93.6% 3344.00 Total 100.0%  100% 3572.00

[0049] In preparing the composite blends disclosed in Table 3 and 4, afirst solution is formed by mixing the thermoplastic binder with water.The first solution is subsequently mixed with a second solutioncontaining water, dispersant, and graphite powder. When mixed together,the first and second solutions form a moist, formable paste. The pastecan be shaped to form a tool mold having a desired configuration.

[0050] In a temperature range between 100° C. to 180° C., samplesprepared from the composite blend shown in Tables 3 and 4 were measuredto have a coefficient of thermal expansion of 9×10-6 mm/mm° C. However,slight shrinkage in the size of the samples occurred in a temperaturerange from between room temperature to 180° C. In order to eliminateshrinkage and obtain dimensional stability in the final tool mold, thetool mold can be subjected to an annealing treatment at the final curetemperature. For example, the samples were annealed at 190° C. for 1hour. After annealing, samples prepared from the composite blend shownin Tables 3 and 4 were measured to have a coefficient of thermalexpansion of 1.81×10-6 mm/mm° C. The coefficient of thermal expansion ofInvar, a conventional tooling material, is reported to have acoefficient of thermal expansion of 1.3×10-6 mm/mm° C. at 23° C. Asindicated, samples prepared from the composite blend shown in Tables 3and 4 have a coefficient of thermal expansion that is comparable toInvar, while having a density of that is one order of magnitude less.

Example 3

[0051] This example illustrates formation of a mandrel and its abilityto be machined. A mandrel, as shown in FIG. 2, has a specific gravity of0.3 (dry) and 0.8 (wet). The important properties are shown in Table 5.TABLE 5 Property Value Compressive Strength approximately 700-1000 psiDensity 28.1 lbs/ft³ (wet) 23.1 lbs/ft³ (dry) Coefficient of ThermalExpansion 6 × 10⁻⁶ in/in ° C.

Example 4

[0052] This example illustrates a formulation that is castable and has ashelf life of approximately 30-45 minutes. This formulation is suppliedin powder form. A typical formulation is shown in Table 6. As shown inTable 6, the formulation contains relatively little binder to provide aless-moisture sensitive formulation. The formulation is mixed with waterin a 3:2 ratio and cast into molds A CTE measurement showed a value ofapproximately 5×10⁻⁶ mm/mm° C. The density of this formulation, 31.8lbs/ft³, was higher than the formulation used in Example 3. TABLE 6 Wt.(g.) Wt. % Plaster of Paris 92.50 37.00% Ceramic microspheres 150.0060.00% PVP 7.50 3.00% Total 250.00 100.00%

Example 5

[0053] This example illustrates use of graphite/coke particles in thecomposite blend. An optimization of the graphite/coke particle sizes andtheir distributions was undertaken to improve the thermal conductivityof the water-soluble formulations. A compatibilizer was used to improvethe dispersion of the graphite particles in water and resin.

[0054] The composite blend includes about 3 wt % PVP, about 39.55 wt %graphite particles, about 39.55 wt % coke particles, about 0.9 wt %lignosulfonate, and about 17 wt % water. Equal amounts of 44 μm graphiteand 450 μm needle coke are used, where these are individual particlesizes. The individual particle size distributions for the graphite andcoke are as follows:

[0055] ˜44 μm Graphite (Type 4012 and Type A625 from Asbury Graphite)

[0056] 61.4%<44 μm

[0057] 26.4%>44 μm

[0058] 12.0%>75 μm

[0059] 0.2%>150 μm

[0060] ˜450 μm Needle Coke (Type 9019 from Superior Graphite Co.)

[0061] 2.78%<150 μm

[0062] 1.97%>150 μm

[0063] 13.32%>180 μm

[0064] 37.95%>250 μm

[0065] 43.59%>425 μm

[0066] 0.39%>850 μm

[0067] Numerous modifications and variations may be made in thetechniques and structures described and illustrated herein withoutdeparting from the spirit and scope of the present invention. Thus,modifications and variations in the practice of the invention will beapparent to those skilled in the art upon consideration of the foregoingdetailed description of the invention. Although preferred embodimentshave been described above and illustrated in the accompanying drawings,there is no intent to limit the scope of the invention to these or otherparticular embodiments. Consequently, any such modifications andvariations are intended to be included within the scope of the followingclaims.

What is claimed is:
 1. A material system for preparing a mold core usedin the fabrication of composite parts comprising: a matrix compositionfor forming the mold core, the matrix composition comprising a watersoluble thermoplastic binder selected from the group consisting ofpolyvinylpyrrolinone, copolymers of polyvinylpyrrolinone, andcombinations thereof; and a finishing composition for smoothing an outersurface of the mold core by covering any undesired surface contours orcracks on the outer surface, the finishing composition comprising awater soluble thermoplastic binder and a hardening compound.
 2. Thematerial system of claim 1, wherein the matrix composition includes anadditive selected from the group consisting of microspheres, hardeningcompounds, talc, metal particles, polyester fibers, polypropylenefibers, graphite particles, coke particles, compatibilizers, dispersantsand combinations thereof.
 3. The material system of claim 1, wherein themold core has a porosity of between about 5 to about 15%.
 4. Thematerial system of claim 1, wherein the matrix composition includesabout 3% thermoplastic binder, about 79.1% graphite and coke particles,about 0.9% compatibilizer and about 17% water all based on the weight ofthe composition.
 5. The material system of claim 1, wherein thefinishing composition includes between about 2 to about 10% watersoluble thermoplastic binder based on the weight of the composition andbetween about 25 to about 50% hardening compound based on the weight ofthe composition.
 6. A composite blend for preparation of toolingmaterials for fabricating composite parts consisting essentially of:polyvinylpyrrolinone, copolymers of polyvinylpyrrolinone, andcombinations thereof; and an additive composition for enhancing thefunctional properties of the blend selected from the group consisting ofpolymeric microbeads, ceramic microbeads, metallic microbeads, hardeningcompound, talc, polyester fibers, polypropylene fibers, metallicfillers, ceramic fillers, compatibilizers, dispersants, and combinationsthereof.
 7. A method for manufacture of a mold core comprising: (a)preparing a core composition having a polymer binder selected from thegroup consisting of polyvinylpyrrolinone, copolymers ofpolyvinylpyrrolinone and combinations thereof; (b) depositing thecomposition in a mold form for shaping the mold core; and (c) drying themold core to remove residual water.
 8. The method of claim 7 furtherincluding the step of machining the mold core to provide a mold corehaving a predetermined shape.
 9. The method of claim 7 further includingthe step of applying a finishing composition to an outer surface of themold core to provide a smooth surface on the outer surface.
 10. Themethod of claim 7, wherein the finishing composition includes a polymerbinder and a hardening compound.
 11. The method of claim 10, wherein thefinishing composition has a viscosity between about 10⁶ to about 10⁷ cPand maintains its positioning on the surface where applied.
 12. Themethod of claim 7 further including the step of forming a composite parton the mold core.
 13. The method of claim 12 further including the stepof removing the mold core from the composite part by solubilizing themold core with a solvent.
 14. The method of claim 13, wherein thesolvent includes water.
 15. The method of claim 13, wherein the moldcore and composite part are cured before the mold core is removed. 16.The method of claim 15, wherein the mold core and composite part arecured at temperatures of up to at least about 550° F.